Vehicles, e.g., automobiles, generally have four wheels; i.e., two front wheels and two rear wheels, and can have an electronic control device for driving the wheels.
Japanese Laid-open Patent Application (JP-A) No. 2006-256605 discloses a four-wheel-drive electronic control unit (4WD-ECU) as such an electronic control device. The 4WD-ECU disclosed in Japanese Laid-open Patent Application No. 2006-256605 together with a vehicle stability assist (VSA)-ECU controls the drive force that operates on the vehicle; and, specifically, sets the four-wheel-drive force in terms of units of, e.g., torque.
Japanese Laid-open Patent Application (JP-A) No. 07-186758 discloses a drive force distribution control device 4 (controller 58) for controlling drive force distribution produced by a drive force transmission system 3 as an electronic control device.
The drive force distribution control device 4 (entire flowchart shown in FIG. 6) disclosed in JP 07-186578 A includes drive force distribution correction means (steps S2, S3, S6, S13, S16 of FIG. 6). The drive force distribution correction means is capable of correcting control signals for adjusting the drive force distribution between main and auxiliary drive wheels for the front and rear wheels in accordance with the detection values of the change-in-rotational-state detection means (deviation of the rotational state detection value of each front and rear wheel corresponding to a main drive wheel and a auxiliary drive wheel). Specifically, when a rapid increasing change has been detected after rapid decrease in the absolute value of the deviation in the detection values of the rotational state of the front and rear wheels, the drive force distribution correction means corrects the change amount of the control signal for adjusting the drive force distribution between main and auxiliary drive wheels for the front and rear wheels in order to reduce the drive force distributed to the main drive wheels. At this time, the state of distribution of the drive force between the front and rear wheels is made the same as the state immediately before the reduction in the absolute value of the deviation in the detection values of the rotational state of the front and rear wheels, and alternatively the ratio of drive force distribution to the auxiliary drive wheels is made to be slightly less than the state immediately before the above-noted reduction, so that the amount of reduction in the drive force to the auxiliary wheels is essentially reduced.
Thus, the inventors recognized that the drive force to the main drive wheels may remain at a high level even if the amount of reduction in the drive force to the auxiliary drive wheels is made less. In other words, the inventors recognized that the main drive wheels may slip in the particular case that the drive force to the main drive wheels was originally high, even if the amount of reduction in the drive force to the auxiliary drive wheels is made less. It is this phenomenon that has not been formally known to those skilled in the art.